332
Alternative Fuels for Transportation
provide sufficient conductivity at low humidity. A mechanical reinforcement
will be useful to sustain the integrity of the membrane under high-mechanical stress during fuel-cell operation. It will be necessary to understand the
location and transport of the water inside the fuel-cell to assess the influence of material properties on performance and durability. Careful fuel-cell
experiments with mathematical simulations will be valuable for the development of new and successful membrane materials.
Current fuel cell stack designs require use of deionized water for the stack
coolant. If the ionic conductivity of the coolant is too high, shunting currents
can arise that lower the stack efficiency. Since deionized water is corrosive
to aluminium radiators, a separate liquid–liquid heat exchanger is required.
Deionized water flows through the stack at one side of the heat exchanger,
while glycol coolant flows between the radiator and the other side of the heat
exchanger. Use of an intermediate heat exchanger further exacerbates removal
of waste heat from the stack. Development is underway on alternative radiator
designs, nonconductive coolants, and stack designs capable of using water/
glycol coolants. Operation at subfreezing temperatures is an issue as pure
water freezes at 0°C. Interestingly, the membrane may not be a significant
problem at subfreezing temperatures as the membrane retains significant proton conductivity even at temperatures of < –20°C. However, the presence of
deionized water will require significant system level developments to match
the performance of current ICEs at low temperatures (Yang 2000).
11.4.1.2 Stack Fuel Delivery Subsystem
For a direct hydrogen subsystem, the fuel delivery subsystem is considerably
more simple than the fuel processor based system (Masten and Bosco 2003).
Hydrogen sources include compressed gas, cryogenic liquid, and metal or
chemical hydrides. In all cases, hydrogen is delivered from the source to the
anode, often with external humidification and temperature preconditioning.
In direct hydrogen applications, where the hydrogen is nearly pure, hydrogen
utilization can be near unity, markedly enhancing system efficiency. Hydrogen
is often recirculated with a pump or ejector to maintain flow and distribution
within the anode. Alternatively, the anode may be operated “dead-ended”
where there is no continuous exhaust flow and only an occasional purge is
required to alleviate contaminant build-up. In the standard case, humidification is achieved through deionized, water-fed membrane humidifiers wherein
a small pump recirculates the anode flow with a solenoid valve and residual
hydrogen is consumed in a small catalytic combustor.
11.4.1.3 Stack Air Delivery Subsystems
A compressor or blower delivers air in excess to the cathode, typically with
external humidification and temperature conditioning. Delivery pressure
Alternative Fuels for Transportation
provide sufficient conductivity at low humidity. A mechanical reinforcement
will be useful to sustain the integrity of the membrane under high-mechanical stress during fuel-cell operation. It will be necessary to understand the
location and transport of the water inside the fuel-cell to assess the influence of material properties on performance and durability. Careful fuel-cell
experiments with mathematical simulations will be valuable for the development of new and successful membrane materials.
Current fuel cell stack designs require use of deionized water for the stack
coolant. If the ionic conductivity of the coolant is too high, shunting currents
can arise that lower the stack efficiency. Since deionized water is corrosive
to aluminium radiators, a separate liquid–liquid heat exchanger is required.
Deionized water flows through the stack at one side of the heat exchanger,
while glycol coolant flows between the radiator and the other side of the heat
exchanger. Use of an intermediate heat exchanger further exacerbates removal
of waste heat from the stack. Development is underway on alternative radiator
designs, nonconductive coolants, and stack designs capable of using water/
glycol coolants. Operation at subfreezing temperatures is an issue as pure
water freezes at 0°C. Interestingly, the membrane may not be a significant
problem at subfreezing temperatures as the membrane retains significant proton conductivity even at temperatures of < –20°C. However, the presence of
deionized water will require significant system level developments to match
the performance of current ICEs at low temperatures (Yang 2000).
11.4.1.2 Stack Fuel Delivery Subsystem
For a direct hydrogen subsystem, the fuel delivery subsystem is considerably
more simple than the fuel processor based system (Masten and Bosco 2003).
Hydrogen sources include compressed gas, cryogenic liquid, and metal or
chemical hydrides. In all cases, hydrogen is delivered from the source to the
anode, often with external humidification and temperature preconditioning.
In direct hydrogen applications, where the hydrogen is nearly pure, hydrogen
utilization can be near unity, markedly enhancing system efficiency. Hydrogen
is often recirculated with a pump or ejector to maintain flow and distribution
within the anode. Alternatively, the anode may be operated “dead-ended”
where there is no continuous exhaust flow and only an occasional purge is
required to alleviate contaminant build-up. In the standard case, humidification is achieved through deionized, water-fed membrane humidifiers wherein
a small pump recirculates the anode flow with a solenoid valve and residual
hydrogen is consumed in a small catalytic combustor.
11.4.1.3 Stack Air Delivery Subsystems
A compressor or blower delivers air in excess to the cathode, typically with
external humidification and temperature conditioning. Delivery pressure